When you look in your adwords campaign, next to your keywords what exactly does it say? Does it say something like, "Raise your bid to $10.00 or raise the quality of your keyword?"
If those keywords have a low quality score, then they charge more. Something similiar happened to me, my affiliate manager e-mailed me with this new idea he had about getting traffic from targetted keywords for a highly competitive offer.
Well, I spent a while on getting all these keywords up, only to find out that while nobody is bidding on these keywords, that I have to pay $5 a click. To get more information you can chat with them live if you login to your adwords campaign, you'll find a link on the page when you add new keywords. So that's what I did, and that's what they told me if the keyword quality is low then you have to pay more, which doesn't make any sense.
I think they're starting to go after charging more for targeted traffic than broad traffic. I'm sure from all these forums, they've found out about how targetted traffic can make affiliates money, while their "big money spenders" are bidding crazy on broad keywords but missing sales due to targetted keywords which are at min. bid.
Edit: Mine were phrases, not a 1 word keyword. Although it was a very specific phrase, and I did this for about 200 items in the store.